The present invention relates to a process for producing quantum fine wires, it is called silicon nanowires, too, and particularly to a process of allowing silicon quantum fine wires to grow on the surface of a silicon substrate at a lower temperature.
A material in the form of quantum fine wires can exhibit new physical properties different from the physical properties inherent to the same material in the bulk form by the nanometer size effect. For example, as shown in FIG. 4, for a silicon (Si) quantum fine wire, the band gap increases with a decrease in diameter. Also, the material in the form of quantum fine wires has a direct transition type band gap, although the same material in the bulk form has an indirect transition type band gap. Consequently, in the silicon quantum fine wires, the re-combination luminous efficiency of excited electrons-positive holes significantly increases, and the luminous wavelength is shifted on the short-wavelength side, which enables emission of visible light.
The silicon quantum fine wires exhibiting these physical properties have been produced by a process of etching a silicon substrate using electron beam lithography or the like. Such a process, however, has a difficulty in production of silicon quantum fine wires, having a uniform distribution of shapes over a wide region, at a high integration.
To solve this problem, there has been proposed a technique in which a large number of silicon quantum fine wires are allowed to grow on a silicon substrate using a VLS (Vapor-Liquid-Solid) process (E. L. Givargizov: J. Vac. Sci. Techno. B11 (2), p449). This technique involves vapor-depositing gold on a silicon substrate and forming drops of a molten alloy of silicon and gold on the surface of the silicon substrate, and heating the silicon substrate while supplying silicon chloride (SiCl.sub.4) gas diluted with hydrogen (H.sub.2) gas as a silicon source gas, whereby allowing the growth of silicon quantum fine wires Wagner et al.: Appl. Phys. Lett. 4, No. 5, 89 (1964); and Givargizov: J. Cryst. Growth, 31, 20 (1975)!.
In the above technique, silicon chloride gas reacts with hydrogen gas, to produce silicon by a thermal decomposition expressed in a chemical equation (1). EQU SiCl.sub.4 +2H.sub.2 .fwdarw.Si+4HCl (1)
The above thermal decomposition is accelerated by the catalytic action of gold on the surfaces of the molten alloy drops formed on the silicon substrate. The silicon thus produced is diffused in the molten alloy drops in a liquid state, and is bonded to solid silicon at the liquid-solid interface. Thus, silicon quantum fine wires grow on the surface of the silicon substrate.
The above related art technique, however, has a problem. Namely, in this technique, the reaction allowing the growth of silicon quantum fine wires must be performed at a temperature being as high as 1000.degree. C. At such a high temperature, silicon chloride is possibly decomposed, though being in a slight amount, on a surface portion of the silicon substrate on which the molten alloy drops are not formed, with a result that each of the silicon quantum fine wires would be formed in a cone shape with a thick root portion. The silicon quantum fine wires such cone shapes cannot exhibit the uniform quantum confinement effect over the entire silicon quantum fine wires.